Potential of lacustrine alkenones as a novel proxy for spring temperatures in mid-latitude European lakes

Past temperature records are key tools for inferring climate dynamics and provide empirical data for testing climate models to improve our mechanistic understanding of natural climate variability. Unfortunately, very few quantitative records of pre-historic continental temperatures exist in Europe. Moreover, existing paleothermometers mainly provide mean annual or warm season temperatures, limiting our understanding of climate variability during the transitional seasons and winter. Alkenones are temperature-sensitive lipids produced by Isochrysidales algae, which have been used for decades to reconstruct quantitative changes in sea-surface temperatures. In lakes, they are not ubiquitous, but they have been increasingly reported in both saline and freshwater lakes worldwide, suggesting that there is great potential for alkenone-based paleotemperature reconstructions in lacustrine settings. Lacustrine alkenones have already been successfuly used to reconstruct paleotemperatures in high-latitude lakes. Depending on the timing of ice-out, they record winter/spring or summer temperatures. In our study, we found that a significant number of Swiss lakes contain lacustrine alkenones. Other studies in mid-latitude European lakes suggest that the peak of alkenone production occurs in spring. The monitoring of Lake St Moritz, an alpine lake in the South East of Switzerland, will allow determining the seasonality of alkenone production in mid-latitute high altitude lakes. Combining genetic analyses and the monitoring of physico-chemical parameters will provide more information about the ecology of the alkenone producers. Our first results suggest that we will be able to improve the understanding of alkenone production in freshwater lakes and to develop the first spring lake temperature reconstruction in Switzerland that extends beyond existing historical records.